Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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SECTION ONE • General Principles

Figure 1-14.  Torque-to-failure test results of rabbit tibiae obtained at 21 to 56 weeks after stabilization with an exter- nal fixator. Large angular displacements at low torque val- ues represent early callus with low torsional stiffness. The torsional stiffness remained low until week 20, and rap- idly increased until week 30. (Reprinted with permission from White AA 3rd, Panjabi MM, Southwick WO. The four biomechanical stages of fracture repair. J Bone Joint Surg Am . 1977;59(2):188–192.)

The biomechanical properties of the natural bone healing progression described above were investigated in a rabbit tibia fracture model. The torsional stiffness and strength of a heal- ing osteotomy were tested at several time points after fixation with an external fixator. 220 A relatively flexible callus was noted initially, with progressively increasing stiffness as the callus matured over time (Fig. 1-14). When loaded to failure, the loca- tion of failure progressed from a refracture through the original osteotomy site at early time points to fracture through bone far away from the osteotomy as healing progressed. Natural bone healing with callus formation benefits from a mechanical environment that allows controlled axial motion, whereby a less rigid fixation construct can yield more callus and a faster callus maturation (Fig. 1-15). The optimal amount of interfragmentary motion is in the range of 0.2 mm to 1 mm, 55,223 although displacements of 4 mm are compatible with healing of tibia fractures treated using braces. 185 Fractures with greater

the intramedullary callus, as the bending stiffness increases with the fourth power as the callus is deposited further away from the neutral axis of the bone (Fig. 1-13C). As stability increases, motion at the fracture decreases, allowing stronger, more mature lamellar bone to form. 10 The fracture will begin to remodel as the mechan- ically strong mature bone is able to bear more load and the large volume of callus is no longer required for stability. The transition from the healing phase to the remodeling phase of fracture healing results in a visible decrease in extra- medullary callus on radiographs. The callus index is the ratio of the diameter of the callus to the diameter of the cortex. 78 As cal- lus grows, the callus index will increase, indicating that healing is progressing. Because remodeling of the fracture callus does not commence until sufficient stability of the fracture has been achieved, the time taken to reach a maximum callus diameter or “maximum callus index” is a useful measurement of the time to clinical union.

A, B

C

Figure 1-15.  Progression of natural bone healing. A: For healing of a 3-mm osteotomy in sheep tibiae, callus formation peaks after 6 to 8 weeks post surgery. (Reprinted with permission from Bottlang M, Tsai S, Bliven EK, et al. Dynamic Stabilization with Active Locking Plates Delivers Faster, Stronger, and More Symmetric Fracture-Healing. J Bone Joint Surg Am . 2016;98(6):466–474.) B: Subsequent remodeling is evident by a decrease in callus size and by a rapid increase in stiffness. Compared to more rigid fixation, less rigid fixa- tion can result in significantly more callus and faster recovery of stiffness. (Reprinted by permission from Springer: Kenwright J, Goodship AE. Controlled mechanical stimulation in the treatment of tibial fractures. Clin Orthop Relat Res . 1989;(241):36–47). C: In humans, progression of fracture healing has been measured by revision of nonunions with an instrumented plate that traced the elasticity across the fracture site over time, whereby 100% and 0% elasticity corresponds to an unstable and fully healed fracture, respectively. In a patient with an undisturbed healing response (labeled “fast healing”), healing of a revised nonunion was achieved within 12 to 16 weeks. (Data from Kienast B, Kowald B, Seide K, et al. An electronically instru- mented internal fixator for the assessment of bone healing. Bone Joint Res . 2016;5(5):191–197.)

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